If you choose to cast a card as Colossal Dreadmaw, while on the stack, treat that spell as though it were Colossal Dreadmaw (including the mana cost to cast it). After it resolves, treat the permanent it becomes as though it were Colossal Dreadmaw for as long as it remains on the battlefield. Once it leaves the battlefield, the card goes back to its normal self.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.

Playtest cards use a modified version of game symbols, such as and . These modified symbols should be treated as the standard symbols during play.

Once you’ve started casting a spell as Colossal Dreadmaw, it’s too late . . . for an opponent to prevent you from doing that by killing The Colossal Dreadmaw.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Official text for Colossal Dreadmaw can be found using the Gatherer card database at Gatherer.Wizards.com.